High frequency amplifier



March 22, 1938. A BRADEN 2,1l2fi65 HIGH FREQUENCY AMPLIFIER Filed Nov. 30, 1935 2 Sheets-Sheet l umHF mi,

INVENT'GH Rene A,Bra den VVitness ZMZfifiSA 2 Sheets-Sheet 2 INVENTOH Heme H. radian FREQUENCY IN hf-C.

N E D A R B A. R.

HIGH FREQUENCY AMPLIFIER Flled Nov 30, 1935 Ma. d?

9 5 C 5 0 g a 1. .w w. v. J

Patented Mar. 22, 1938 e TET OFFICE" HIGH FREQUENCY AMPLIFIER.

Rene A. Braden, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 30, 1935, Serial No. 52,317

5 Claims.

such a wide frequency band that it is difiicult to obtain satisfactory amplification over the entire band. At the high frequency end of the band, especially, the gain of an amplifier drops very rapidly unless special precautions are taken.

An additional difiiculty is encountered when a vacuum tube having a control grid is employed as a final detector since it is necessary to: filter out the carrier frequency and, at the same time, prevent the gain of the amplifier from dropping too rapidly at the high frequency end.

It is, accordingly, an object of my invention to provide an improved high frequency. amplifier in which the gain is satisfactory at the high frequency end of the band of frequencies being amplified.

A further object of my invention is to provide an improved detector and amplifier system in which the carrier frequency is filtered out while maintaining the gain at the high frequency end of the band at a satisfactory level.

In a preferred embodiment of my invention I provide in the output circuit of the second detector of a superheterodyne receiver a low pass filter section so designed that it filters out the carrier frequency. While such a filter section is very satisfactory for removing the undesired carrier frequency it tends to reduce the gain of the receiver at the high frequency end. In order to hold up the gain at the high frequencies I provide an inductance coil on each side of the filter section and in series with the filter coils, these added coils having much greater inductance than the filter coils whereby they are effective at the picture frequencies.

Other objects, features, and advantages of my invention will appear from the following description taken in connection with the accompanying drawings in which Figure l is a circuit diagram of a portion of a television receiver embodying my invention,

Figures 2, 3, and 4 are circuit diagrams showing other embodiments of my invention and Figure 5 is a group of curves which are referred to in explainingmy invention.

Referring to Figure 1, my invention is shown applied to a television receiver of the superheterodyne type, the second detector being indicated at l and the first picture signal amplifier tube being indicated at 2. The detector tube I is of the screen grid type having a cathode 3, a control grid 4, a screen grid 5 and a plate T. A self -biasing resistor 8 and. by-pass condenser 9 are provided for maintaining the control grid 4 at a suitable negative bias. The bias is applied to the control grid 4 through a grid resistor II and the inter- 5 mediate frequency signal is impressed upon the grid 4 through a coupling condenser l2.

The amplifier tube 2 also may be of the screen grid type having a cathode I3; a control grid M, a screen grid l6 and a plate 11. As in the case of the detector tube, the control grid l i'may be biased negatively by means of the usual self-bias resistor i3 and by-pass condenser IS, the negative bias being applied to the grid through a grid resistor 2!. It will be understood, of course, that the bias applied to the grid I4 of the amplifier tube 2 is considerably less than that applied to the grid of the detector tube. The plate I! of the tube 2 is supplied with a positive potential through a plate resistor 22 and peaking coil 23 from any suitable source, the peaking coil 23 being provided to hold up the gain of the amplifier at the higher picture frequencies.

The plate 1 of the detector tube I is coupled through a pair of filter coils 24 and 26 and a coupling "condenser 21 to the input circuit of the amplifier tube 2. A condenser 28 is connected between ground and a point between the filter coils; 24 and in order to form a low pass filter section which is very effective in filtering out the intermediate'frequency carrier. As Will be seen by referring to the curve D in Figure 5, the inser tion of the filter section in the detector output circuit without the addition of otherinductance coils would cause the response curve of the amplifier to drop very rapidly at the higher picture frequencies. Therefore, I pro-vide two inductance coils 29 and 3i connected in series with the filter coils 2 and 26, the inductance coil 29 being located between the plate I of the detector and the filter section and the other inductance coil 3| being located between the filter section and the coupling condenser 21.

The filter coils 24 and 26 have a comparatively small inductance, 50 microhenries in the specific circuit being described, since they are to be effective at the high frequency of the carrier wave rather than in the much lower frequencies of the picture signals. The filter condenser 28 may have a capacity of 20 micro-microfarads as indicated on the drawings, the capacity of this condenser usually having a value which lies somewhere between the value of the anode to ground capacity of the detector and a capacity of about twice this value. It will be understood that the filter capacity may be the capacity of the filter coils to ground instead of a separate condenser 28.

The inductance coils 29 and 3| have a comparatively high inductance value since they are to be effective at the picture frequencies, this value being 5 millihenries in the circuit illustrated.

The plate I of the detector tube is supplied through a plate resistor 32 with an operating voltage from any suitable source (not shown) which is by-passed by a suitable by-pass condenser 33. In the embodiment shown in Figure 1, the plate resistor is connected to a point between the inductance coil 3! and the coupling condenser 27 whereby plate voltage is fed through the two coils 29 and 3| and the filter coils 24 and 26. This results in an amplifier which is entirely satisfactory, the frequency response characteristic of the amplifier being shown by the curve A in Figure 5. The peaks and dips in the curve at the higher frequencies are not harmful since the amplifier has a suitable delay characteristic at these frequencies. It should be understood, however, that it is undesirable to connect several amplifiers having this characteristic in cascade as the characteristic would then be unduly exaggerated. If desired, the peaks may be flattened or lowered in amplitude by inserting resistance in series with the inductance coils.

A still better amplifier characteristic can be obtained by connecting the plate resistor 32 direct-- ly to the plate I of the detector tube as shown in Figure 2. In Figures 1 and 2, similar parts are indicated by the same reference numerals. The characteristic of the amplifier circuit shown in Figure 2 is indicated by the curve B in Figure 5. It will be seen that the peaks and dips in this curve are less pronounced than in the curve A.

Referring to the embodiment of my invention shown in Figure 3 the plate resistor may be divid' ed, one plate resistor section 32a being connected to a point between the coil 3| and the coupling condenser 21 and the other plate resistor section 32b being connected to a point between the coil 29 and the detector plate 1. Like parts in Figures 1 and 3 are indicated by the same reference numerals. The frequency response curve for the amplifier shown in Figure 3 is indicated by the curve C in Figure 5. It will be seen that this amplifier has a smoother curve than those of the preceding amplifiers.

Referring to Figure 4 in which parts similar to those in Figure 1 are indicated by the same reference numerals, I have illustrated the split plate resistor feature applied to an amplifier which does not have the low pass filter section in its circuit. In this circuit, a vacuum tube 35, which may function as a non-distorting amplifier rather than as a detector, has an inductance coil 31 connected between its plate 38 and the coupling condenser 21. The coil 31 has an inductance value such that it holds up the gain of the amplifier tube 36 at the high frequencies. As in the circuit shown in Figure 3, the plate voltage is fed to the plate of the tube 36 through two plate resistors 39a and 39b, one of which is connected to the anode side of the inductance coil 31 and the other of which is connected to the coupling condenser side of the coil 31. It will be seen that the two plate resistors 39a. and 392) are connected across the coil 31 in series whereby they act as a damping resistor to prevent sharp resonant peaks.

In Figs. 1 to 4 the Values of the plate resistors are indicated in ohms, these values being given merely by way of example.

It will be apparent from the foregoing description that various modifications may be made in my invention without departing from the spirit and scope thereof and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and are set forth in the appended claims.

I claim as my invention:

1. In a receiver for the reception of a carrier wave modulated by signals covering a certain frequency band, a detector comprising an electric discharge tube having a cathode, an anode, and a control grid, means for impressing a carrier wave modulated by said signals across said grid and cathode, means for supplying a positive voltage to said anode, an amplifier tube having an input circuit, means for coupling said detector anode to said input circuit through a first coil, a pair of filter coils and a second coil connected in series in the order named, and a filter condenser connected between ground and a point between said filter coils, said filter coils and said filter condenser having inductance and capacity values such that they filter out the carrier frequency, and said first and second coils having inductance values such that they hold up the gain of the receiver at the high frequency end of the band of modulating frequencies.

2. The invention according to claim 1 characterized in that a positive voltage is supplied to said detector anode through a plate resistor connected to a point between said detector anode and said first coil.

3. The invention according to claim 1 characterized in that a positive voltageis supplied to said detector anode through a plate resistor and all of said coils in series.

4. The invention according to claim 1 characterized in that a positive voltage is supplied to said detector anode through a plate resistor connected to a point between said detector anode and said first coil and also through a second plate resistor connected to that end of said second coil which is not connected to a filter coil.

5. In an amplifier for high frequency signals covering a certain frequency band, an electric discharge tube having an input circuit and having an output circuit including an anode, a utilization circuit, means for coupling said output circuit to said utilization circuit through an inductance coil and a coupling condenser in series, and means for supplying a positive voltage to said anode through two plate resistors, one of said resistors being connected to a point between said anode and said coil, and the other of said resistors being connected to a point between said coil and said coupling condenser, said coil having an inductance value such that the gain of said amplifier tube is held up at the high frequency end of said certain band.

RENE A. BRADEN. 

